The bacitracins, polymyxins and EM 49 are peptide antibiotics which affect the structure and function of bacterial membranes by interacting with membrane lipids. The long term goal of the proposed research is to elucidate the mechanisms for the antimicrobial activities of these antibiotics and to characterize their interactions with membrane lipids. Bacitracin A inhibits the biosynthesis of peptidoglycan by forming a strong complex with the C55 isoprenyl pyrophosphate and divalent cation. The structure of this complex will be characterized using NMR and ESR techniques. Polymyxin B and EM 49 affect the permeability of bacterial membranes and phospholipid liposomes. The influence of these antibiotics on the transmembrane electrochemical and proton gradients of bacterial membranes will be examined using flow dialysis. In addition, the effects of polymyxin B and EM 49 on the structure of phospholipid bilayers in bacterial membranes and liposomes will be studied using fluorescence, NMR and ESR techniques. EM 49 contains a fatty acid covalently linked to the peptide through an amide linkage. This fatty acid has been replaced by a fatty acid containing a nitroxide spin label and this derivative will be used to determine whether or not the peptide fatty acid is inserted into the hydrophobic core of bacterial and liposome membranes. This study should provide valuable information concerning the molecular basis for the biological properties of these antibiotics and the chemistry of lipid-peptide interactions.